Certain hard to earn (or time consuming) achievements could reward special pieces of cosmetic gear. For instance, if you complete all jumping puzzles, you could get something looking like Hermes boots. Something similar to WoW achievement tabards (ok they were pretty lame in most cases, but still better than nothing).
If they ever implement player housing, this could be extended to house furniture or decorations (like big globe with map of tyria on it for completing the map).
Lack of cosmetic rewards in GW2 is astonishing...

For you and me, Anet seems to have changed course on the whole skins thing really, even the Halloween and likely Wintersday styles were completed prior to release, along with all the skins including the underwater skins. I don't see anything being added that would refute that claim.

Oh please no. I hate the idea of "achievement points" in general merely because you can get infinite amount of them if you really want to which makes them rather pointless.

Good point, but keep in mind that most people gain their achievement points without using the infinite salvage achievement. I have no clue why they decided to put something like that in the game but at the moment having thousands of achievement points means that you spend a lot of time playing the game - something that should be rewarded.